The present invention pertains guest-host liquid crystal mixtures wherein the guest is a dichroic fluorescent dye.
Liquid crystals (LC) are well known to those skilled in the art and can be defined as that class of matter which has an intermediate or mesomorphic state in which these substances behave mechanically as liquids yet exhibit many optical properties of crystals. The mesomorphic state or liquid crystalline phase is obtained either by heating liquid crystals that are in the solid phase or by cooling liquid crystals that are in the liquid phase. Liquid crystal substances may exist in one of the cholesteric, smectic or nematic state. The cholesteric state is distinguished from the nematic state by its markedly different optical properties.
For example, cholesteric materials are optically negative whereas nematic materials are optically positive. Optically positive liquid crystals transmit light more slowly perpendicular to the layers of molecules than parallel to them. Nematic liquid crystals orient a beam of ordinary light into two polarized components whose transverse vibrations are at right angles to each other. However, application of force field, such as an electric field, lines the molecules up, changing their optical properties.
In the past liquid crystal “guest-host” display devices have been developed. The term “host” refers to the liquid crystal material, and the term “guest” refers to an agent which may be aligned by the host to produce contrasting light absorptive states in response to selectively activeable external stimuli. For example, U.S. Pat. No. 3,833,287 to Taylor, et al. discloses a host nematic liquid crystal used in conjunction with a pleochroic guest material which in mixture corresponds to the helical molecular orderings of the nematic liquid crystal material. In the helical ordering the guest material is absorptive of incident light, whereas the guest material is non-absorptive of light when aligned by the nematic crystals under the influence of an applied electric field. Accordingly, when an electric field is applied to a thin layer of the mixture in the display device, incident light passes through the mixture and is reflected off the backing thus exhibiting the color of the backing.
In recent years refinement of effective guest-host systems have been developed using dichroic dyes. Dichroism is the property whereby an oriented assembly of dye molecules exhibits relatively low absorption of a given wavelength of light in one state of orientation and a relatively high absorption of the same wavelength in another state of orientation with respect to the light source. The orientation can be brought about by dissolution of the dye in a liquid crystal solvent or by embedding the dye in a stretched plastic.
Dichroic fluorescent dyes have applications in many fields, for example in displays [Chen and Swager: Org. Lett., Vol. 9, No. 6, 2007], lighting [US 2007/0273265], lasers [JP1994318766] and switchable glazing [WO2009141295]. In order to have a liquid crystalline dye mixture within a useful temperature range, it is common to mix these dichroic dyes with thermotropic liquid crystals. These liquid crystal dye mixtures are known as guest-host LC systems, as the alignment of the dye (the guest) is dictated by the alignment of the liquid crystal (the host).
The purpose of these LC-dye mixtures is to be able to control the absorption or fluorescence emission (or both) of light by the dye. A measure of how well the absorption can be controlled is the dichroic ratio of the dye in the LC mixture. The dichroic ratio is the ratio in absorption for light coming in parallel to the absorption axis of the molecule (A∥) versus light traveling perpendicular to the absorption axis (A—|—):
DR=A∥/A—|—. Another way of expressing the same property is the order parameter (S), which is defined as
  S  =                    (                              A            //                    -                      A            ⊥                          )                    (                              A            //                    +                      2            ⁢                          A              ⊥                                      )              .  The order parameter is 1 for perfect order and 0 for a perfect isotropic system. A high dichroic ratio or order parameter allows a high contrast in absorption or emission, which is desirable in many applications.
At the same time, it is desirable to have a high solubility of the dichroic dye in the liquid crystal. The higher the solubility of the dyes, the less liquid crystal is required as host material, offering a potential to save material.
As a third requirement, a high light stability of the dye is desirable. A high light stability allows a long lifetime of the devices in high power conditions such as sunlight, lasers or interior lighting.
As a fourth requirement, the fluorescent dichroic dye should have high quantum efficiency. The quantum efficiency is the ratio of emitted photons to absorbed photons of the dye. The higher the quantum efficiency of the dye, the better the dye fluoresces. A good fluorescent dye has a quantum efficiency of at least 0.5, but preferably close to 1.
Many classes of dyes have been proposed which are both fluorescent and dichroic. A general introduction to dichroic dyes can be found for example in Liquid crystals: applications and uses, Volume 3, by Birenda Bahadur (Edt), World Scientific Publishing, 1992.
Fluorescent dichroic anthraquinone dyes have been used in display applications, where light intensities were moderate. For examples, an excellent report on the search for fluorescent dichroic dyes in fluorinated liquid crystals is given by Iwanaga, in Materials 2009, 2, 1636-1661; showing dichroic ratios up to 10 with solubility of 1-2 wt % in fluorinated liquid crystal LIXON 5052 XX.
However anthraquinones are limited in their lifetime under high light intensities and have moderate quantum efficiencies.
Good aligning fluorescent dichroic dyes are the Acenequinones type by Chen and Swager: Org. Lett., Vol. 9, No. 6, 2007. Again here the quantum yield is limited (0.30 or lower).
Thiadiazole dyes have been tried, for example by Xuelong Zhang, et al, J. Mater. Chem., 2006, 16, 736-740. There, reasonable (0.5-0.8) quantum efficiencies were found with very good alignment (DR=10-15) for reasonable concentrations (0.5 wt %). The light stability is moderate and does not fulfill the demands for high intensity applications.
Excellent order parameters are achieved when fluorescent dyes self-assemble into wires and are suspended in liquid crystals, see for example Shane Moynihan, Pierre Lovera, Deirdre O'Carroll, Daniela Iacopino, and Gareth Redmond Adv. Mater. 2008, 20, 2497-2502. However, it is difficult to achieve high concentrations while avoiding agglomeration of the aggregates causing quenching of the fluorescence.
The class of rylene type dyes has received particular attention for its excellent fluorescence quantum yield (0.8-1) and its high light stability. Therefore, many people have tried to align these materials in liquid crystal hosts:
U.S. Pat. No. 4,378,302 describes mixtures of perylene bisimide with liquid crystals to form a liquid crystal guest-host mixture.
E WOLARZ, H MORYSON AND D BAUMAN, in Displays, Vol. 13 No 4 1992, pp 171. Shows a maximum order parameter of 0.63 for rylene type dyes.
Perylenes in liquid crystal mixture E7 are shown in Roland Stolarski and Krzysztof J. Fiksinski, Dyes and Pigments 24 (1994) 295-303, reporting an order parameter of 0.41 for perylene-3,9(or IO)-dicarboxylic acid derivatives.
Perylene diester fluorophores in the liquid crystal mixture E7 are reported in: Anisotropic fluorophores for liquid crystal displays, R L VAN EWYK, I O'CONNOR, A MOSLEY, A CUDDY, C HILSUM, C BLACKBURN, J GRIFFITHS AND F JONES in DISPLAYS, OCTOBER 1986, pp 155.
Light fast rylene type dyes are commercially sold by BASF under the name Lumogen. Their order parameter in a nematic liquid crystal is low:
TABLEmaximumPeak absorptionorder parametersolubility Name(nm)in nematic LC(wt %)Lumogen F yellow 083480.50.330.05Lumogen F yellow 170511.00.450.05Lumogen F orange 240533.00.30Lumogen F pink 285557.00.19Lumogen F red 300574.5−0.09Lumogen F red 305577.5−0.120.2Lumogen F green 850482.50.30Lumogen IR 765712.0 (crystals)0.79 (crystals)0.08Lumogen IR 788725.0 (crystals)0.30 (crystals)Insoluble
As is apparent from the table above, these dyes suffer from a low order parameter. The Lumogen IR 765 and Lumogen IR788 have poor fluorescence and have no significant solubility (<<0.1 wt %) in commercial liquid crystal mixture MLC6653 from (Merck Darmstadt), a nematic liquid crystal mixture with positive dielectric anisotropy and a positive delta n.